Mystery misfire!

[pzorb]

On full boost, when the car's been driving for maybe an hour, I get the whole lungey on/off behaviour that I associate with a misfire. Thing is, iridium sparkies have got 8k on them, same with the rotor arm and dizzy. Cables at least 20k (unchanged since I've had the car, dunno about before). I took the whole lot out for a look last weekend and everything seemed in order.
It doesn't misfire if I do just below WOT; nor WOT after the car's engine is warmed up and on a short ~20 minute journey - so something's happening after a while to cause a misfire. (basically when the boost needle goes off the gauge and a semi long journey) :slap:

Could it be the cables? I didn't change them because a) they cost too much and b) if they work, they work - AFAIK there's no inbetween

Any ideas?
(855, 174k, rica 280, manual)

[Murphy]

Do you have a dump valve ?

Do you run the car low on fuel ?

[BlackBeast]

I think you've proven that the spark is ok, but what about the fuel system. Start with the easiest first, the fuel filter, when was the last time it was changed. Either clean it out or for peace of mind change it for a new one. If nothing improves take the fuel pump out of the tank and check the filter at the end of it, clean it out if it needs it. Then check the voltage at the fuel pump, it might be fine during normal idleing/driving, but as your problem is on boost make sure you check it then. If the voltage is too low then it might be time to upgrade to a walbro etc.

Hope that helps.

[pzorb]

No dump valve, and I always make the most of my fuel capacity!

How often does the fuel filter get replaced according to volvo's schedule? Last 'proper' service it had was at 160k (fluid changes at 5k ever since).

It only ever runs optimax.

Mind you... coming back from plymouth last month it was down on power but was put down to a MAF sensor playing up...

[deerworrier]

was down at the rugby last week and on the journey home was getting strange hesitation under acceleration, checked fuel filter and the bugger was mostly water??? got new one drained out last of crap fill and no probs since:) i would start with the filter and work from there.

[pzorb]

OK filter it is. Had a look at haynes, but it didn't mention anything about gushing petrol... I'm assuing that breaking the fuel line means a spill? Any ideas? Can the line be pinched or is it too solid?

[BlackBeast]

I havent changed the fuel filter on mine yet, but on all other cars ive done them on the fuel system is pressurised when the engine is running, and also for a while after its been switched off, so dont do it straight after the engine has been on, you will get a little petrol spillage though, but not too much.

[Engineer]

If everything else checks out ok it sounds like your overboosting and hitting the cutout mate, check your manual settings...........................cold air more oxygen etc.

[pzorb]

Right OK I've **FINALLY** solved the problem. The mystery misfire wasn't cables, wasn't ignition.... it was....

The MAF cable connector!

Disconnected it, gave it a good filing, shined up the contacts and DISCO :rainfro: we're back in business!

So for anyone out there with a misfire that occurs only at high rpm, check the MAF cable!
(and i was about an hour from ordering a new MAF) Wasn't caught by the ECU error code system either.

I think it was intermittant because of the movement of the cable inside the engine bay. I might tie it down with some cable ties when I have the chance.

Now I only have a slight misfire at very high rpm, but that's almost certainly due to the garage taking out my 8000 mile new iridiums and replacing them with crappy OEM ones.

I'm SO happy I've got boost back!

[Engineer]

Right OK I've **FINALLY** solved the problem. The mystery misfire wasn't cables, wasn't ignition.... it was....

The MAF cable connector!

Disconnected it, gave it a good filing, shined up the contacts and DISCO :rainfro: we're back in business!

So for anyone out there with a misfire that occurs only at high rpm, check the MAF cable!
(and i was about an hour from ordering a new MAF) Wasn't caught by the ECU error code system either.

I think it was intermittant because of the movement of the cable inside the engine bay. I might tie it down with some cable ties when I have the chance.

Now I only have a slight misfire at very high rpm, but that's almost certainly due to the garage taking out my 8000 mile new iridiums and replacing them with crappy OEM ones.

I'm SO happy I've got boost back!
Well done mate. It should be tied down as standard, my guess is most mechs. etc. don't bother if they have removed for whatever reason be it service or repair. Attention to detail pays off.

[Vikingxl]

Glad you got it sorted without having to spend out!

[t5tart]

Well done mate. It should be tied down as standard, my guess is most mechs. etc. don't bother if they have removed for whatever reason be it service or repair. Attention to detail pays off.


mine isnt tied down wonder if thats what mine is doing ....


stupid question but where is the best route to tie it to
any one got any pics ?